This invention generally relates to AC-DC transfer standards.
An AC-DC transfer standard provides a DC output voltage approximately equal to the true RMS value of a complex or sinusoidal AC signal (referred to hereinafter as an AC signal). In one type of transfer standard, the AC signal to be measured is applied to a heater element adjacent to or contacting a temperature sensor.
One type of temperature sensor is a bimetallic junction thermocouple, which comprises two wires of dissimilar metals that are joined at both ends. According to the Seebeck effect, heating one junction will induce a current in the wires. The joule heating caused by the AC signal induces a voltage in the thermocouple, which voltage is proportional to the temperature difference between the bimetallic junction adjacent the heater element and the cold junction temperature.
The thermocouple voltage is nulled by adjustment of a potentiometer, which is adjusted to apply an equal and opposite voltage to that generated by the thermocouple. Then, without changing the potentiometer setting, an easily measured DC voltage is substituted for the AC signal. The DC voltage amplitude is increased until the voltage generated by the thermocouple is again equal and opposite to the voltage generated by the potentiometer, as previously adjusted for the AC signal. When this null condition is reached, the amplitude of the DC voltage is measured. This DC amplitude is equal to the RMS value of the AC signal.
The heater element in such devices is typically a heater wire made of material having a relatively high electrical resistance, such as nickel-chromium alloy or Evanohm.RTM. (the latter being preferred). The leads connected to the heater wire are typically made of material having a relatively low electrical resistance, such as Dumet. As a result of dissimilar metal contact, the junctions between the heater wire and its connecting leads experience a Peltier effect when DC current passes through the heater wire, meaning that one bimetallic junction of the heater wire is heated and the other end is cooled. At the same time, the heater wire experiences a Thomson effect, which causes heat to flow into or out of the heater wire when it is subject to a temperature gradient and current flows through it.
Because of the Peltier and Thompson effects, a sensor measuring temperature at the center of the heater wire, or at any other point on the heater wire, will sense different temperatures for DC currents passed through the heater wire of equal magnitude but of opposite polarity. This error is termed DC reversal error. In order to minimize DC reversal error in making transfer standard measurements, a number of measurements must be taken: first, the AC signal is read and nulled; second, a positive DC voltage measurement is taken; third, the AC signal is again measured and nulled; fourth, a minus DC voltage measurement is taken; and fifth, the AC signal is again read and nulled. The AC signal's RMS value is then computed by summing the amplitudes of the two DC voltage readings and dividing by two. The resultant RMS value is considered valid only if the unknown signal has not significantly drifted, as determined by the three measurements of the unknown signal. When great care is exercised, the best transfer measurement accuracy that is realizable (using a thermocouple as the temperature sensor) with this five step measurement process is an uncertainty of about 10 ppm. An uncertainty of about 25 ppm is generally more typical.
As an alternative to a thermocouple, the temperature of the heater wire can also be sensed with a sensing wire having a high temperature coefficient of resistivity. Typically, the sensing wire is wrapped about and electrically insulated from the heater wire. The electrical resistance of the sensing wire is measured to determine the temperature of the heater wire. (Further particulars concerning this type of sensing device can be found in application Ser. No. 580,450 entitled "Resistive Sensing Thermal Device for Current Measurement," inventor Fred L. Katzmann, filed Feb. 15, 1984 and assigned to the assignee of this application.) This means for sensing the heater wire temperature may eliminate most of the errors caused by the Peltier and Thomson effects; however, the improvement may not be sufficient to allow the user of the AC-DC transfer standard to dispense with the five step measurement process described above for the very highest precision transfer measurements.
As may be easily appreciated, the five step measurement process to determine the RMS value of an AC signal is quite cumbersome. Using this process, it can take perhaps one day to calibrate and certify a precision DC voltmeter. Making only one DC voltage measurement certainly would hasten the process, but, given present techniques, only at the expense of unacceptable measurement error due to DC reversal error in the thermoelement.